Semiconductor lasers have become more important. One of the more important applications of semiconductor lasers is in communication systems where fiber optic communication media is employed. With growth in electronic communication, communication speed has become more important in order to increase data bandwidth in electronic communication systems. Improved semiconductor lasers can play a vital roll in increasing data bandwidth in communication systems using fiber optic communication media.
The operation of basic semiconductor lasers is well known. Semiconductor lasers can be categorized as surface emitting or edge emitting depending upon where laser light is emitted. They may also be classified by the type of semiconductor junctions used such as heterojunction or homojunction Referring to FIG. 1A, a prior art vertical cavity surface emitting laser (VCSEL) 100 is illustrated. VCSEL 100 is cylindrical in shape and includes heterojunctions. When VCSEL 100 is lasing, the laser light is emitted from the top surface in a region defined by the optical confinement region 103. VCSEL 100 includes a first terminal 101 and a second terminal 102 coupled respectively to the top and bottom surfaces of the VCSEL to provide current and power. VCSEL 100 includes distributed Bragg reflector (DBR) layers 104A and 104B defining the optical confinement region 103. The optical confinement region 103 provides optical confinement such that the light can be reflected between the DBR layers 104A and 104B in a reinforcing manner to provide light amplification. VCSEL 100 includes heterojunction layers 105 which forms an active region 106 with the optical confinement region 103. The active region 106 provides current confinement so as to provide lasing when a threshold current is supplied to the VCSEL 100. Threshold current is the current level required for injecting enough carriers (electrons and holes) for lasing to occur. When lasing, the VCSEL 100 has a transverse mode field 108 and a longitudinal mode field 109. To improve optical confinement index guiding may be used. Index guiding uses layers of different compounds and structures to provide a real refractive index profile to waveguide the light. Alternatively a VCSEL may be gain guided. In gain guiding, the carriers induce a refractive index difference which is a function of the laser current level and output power.
There are three types of prior art VCSEL devices that are of interest. These are planar proton implanted VCSELs, ridge waveguide VCSELs (RWG) and oxide confined VCSELs. Referring now to FIG. 1B, a planar proton implanted VCSEL 110 is illustrated. Planar proton implanted VCSELs are relatively easy to fabricate and have a planar top surface that allows easy contact metalization and metal interconnect. As a result, a large contact area can be manufactured with low resistance. However, planar proton implanted VCSELs lack sufficient index refraction difference in the lateral direction to provide good optical confinement. Optical confinement of planar proton implanted VCSELs is generated by gain guiding and thermal lensing effect caused by heating. The thermal lensing effect (diverging/converging) provides a change in the index of refraction as a proportion of temperature due to the heating of the junctions. These methods of optical confinement provide poor performance and result in planar proton implanted VCSELs having a high threshold current, large turn-on delay and large timing jitter. The turn on delay and timing jitter of a VCSEL are functions of the threshold current. The lower the threshold current the easier it is to turn on a VCSEL and the less is the turn on delay time needed to generate the appropriate amount of current with the VCSEL for lasing. The higher the turn on threshold the greater is the timing jitter in turning on and off a VCSEL. The high threshold current additionally implies a higher operation current and thus a shorter lifetime in the operation of the planar proton implanted VCSEL. The planar proton implanted VCSEL having a large turn-on delay and a large timing jitter makes it unsuitable for high speed applications beyond 2.5 Gbps.
Referring now to FIG. 1C, an improvement over planar proton implanted VCSELs is the prior art ridge waveguide proton implanted VCSEL 120. Ridge waveguide VCSELs have stronger optical mode confinement provided by the large index refraction difference between semiconductor and air. This large index refraction is similar to what is provided by edge emitting semiconductor lasers. The ridge waveguide proton implanted VCSEL provides a lower threshold current than a planar proton implanted VCSEL and thus potentially longer operational lifetime. The thermal lensing effect is minimal in ridge waveguide proton implanted VCSELs, and thus have fast turn-on and turn-off times. The timing jitter in ridge waveguide proton implanted VCSELs is much smaller than that of planar VCSELs. The ridge waveguide proton implanted VCSELs can potentially operate up to 5 Gbps, beyond which, they are limited by an RC time constant—the resistance being that of the device. However, ridge waveguide VCSELS are difficult to manufacture because of their nonplanar surface. The top surface metalization of a ridge waveguide VSCEL is particularly difficult to manufacture. A disadvantage to ridge waveguide VCSELs is that the heat dissipation is poor resulting in a thermal resistance typically 50% greater than that of planar proton implanted VCSELs. Thermal resistance causes a temperature rise in the active region as a function of the dissipated power therein. Heat dissipation is a very important factor in improving semiconductor device reliability. Large device resistances result in large RC time constants, ultimately limiting the device from high speed applications.
Referring now to FIG. 1D and FIG. 1E, an improvement over ridge waveguide VCSELs is the prior art oxide confined VCSELs 130 and 140. Oxide confined VCSELs utilize a partially oxidized AlAs layer to provide current blocking for current confinement. Oxide confined VCSELs have lower threshold currents due to good current confinement and lower resistance that allows for high speed operation. Depending on where the oxidized current blocking layer is manufactured in an oxide confined VCSEL, optical confinement for the optical mode of the semiconductor laser can be provided by the index refraction difference between the oxidized portion of the AlAs layer and the non-oxidized portion of the AlAs layer. Typically, Al2O3 has an index of refraction of about 1.5 and AlAs has an index of refraction of 2.9. Disadvantages associated with the oxidized VCSEL technology are difficult manufacturability (i.e., low yield) and poor uniformity, consistency, and reliability. Generally, it is desirable to avoid oxidizing a material within a VSCEL because it creates lattice defects that will grow and eventually degrades VCSEL device performance. In addition, stresses caused by the volume change after material oxidation accelerates VCSEL device degradation.